King's 2020 is a complex issue.
We don't know, but I feel 100% confident that he was playing hurt.
But to your point, he sucked no matter who he was covering last year. And he was not routinely being asked to cover small, quick guys.
There's also the problem that if he's a total liability against smaller, fast receivers, then he's just plain a liability. Because there are a lot of those guys in the league, it isn't that hard for offenses to get them matched up on him, and oftentimes the big guys (e.g. in division-- Golladay, Robinson) are going to eat his lunch too.
And if the solution to that is just to play zone, that doesn't help, because King is a wretched zone corner. He's only remotely passable in man coverage.
And on top of all that, I think he was relatively healthy in 2019, and didn't think he was very good then either.
All of this to say, if the solution at the other outside corner spot is resigning King, that's not very exciting at all, and we can all hope that a rookie beats him out, which shouldn't be hard to do.